Conventional cathode-ray tubes have a colour selection mask located at a precise distance from the inside of the glass front face of the tube, on which front face red, green and blue phosphor arrays are deposited in order to form a screen. The mask consists of a metal sheet perforated in its central part with a plurality of holes or slots. An electron gun, placed inside the tube, in its rear part, generates three electron beams in the direction of the front face. An electromagnetic deflection device, generally placed outside the tube and close to the electron gun, has the function of deflecting the electron beams so as to make them scan the surface of the screen on which the phosphor arrays are placed. Under the influence of the electron beams, each one corresponding to a particular primary colour, the phosphor arrays enable images to be reproduced on the screen, the mask allowing each particular beam to illuminate only the phosphors of the corresponding colour.
The colour selection mask must be placed and held during operation of the tube in an exact position inside the tube. The function of holding the mask is achieved by virtue of a rectangular metal frame, which is usually very rigid, to which the mask is conventionally welded. The frame/mask assembly is mounted inside the front face of the tube by virtue of suspension means usually welded to the frame and cooperating with tabs inserted in the glass forming the front face of the tube.
The current trend is for tubes whose front face is increasingly flat, with a tendency towards completely flat faces. The production of tubes having such a front face involves a technology consisting in using a flat mask, held under tension in at least one direction. Such structures are described, for example, in U.S. Pat. No. 4,827,179.
More conventionally, the mask may be formed by pressing, its surface perforated with openings then being slightly curved in order to follow the inner curvature of the glass front face of the tube. The peripheral skirt of the mask, formed so as to be perpendicular to the surface perforated with openings, is conventionally welded to the edge of the support frame.
Since the colour selection mask consists of a very thin metal sheet, placing it under tension may generate interference in the form of vibration of the said mask during operation of the tube. Under the effect of external impacts or mechanical vibration, for example acoustic vibration due to the loudspeakers of the television set in which the tube is inserted, the mask may start vibrating at its natural resonant frequency. The vibration of the mask changes the region where the electron beams land on the screen of the tube, the point of impact of each beam then being offset with respect to the associated phosphor array, thus creating discolouration of the image reproduced on the screen.
The phenomenon may also occur for a mask formed by pressing, since its virtually flat surface does not have enough mechanical rigidity to be insensitive to the vibration phenomena generated by the tube environment.
U.S. Pat. No. 4,827,179 proposes adding means for damping the vibration of the mask onto one face of the mask. These means are, in the known manner, placed on the peripheral part of the mask not perforated with openings. However, the damping devices used in this patent have a complicated structure which is difficult to implement. These devices must be fitted to the surface of the mask once the latter is secured to the frame, since the fragility of the thin metal sheet perforated with openings forming the mask does not make it possible to fit additional components thereto before it is fitted on the frame. However, here again, the fragility of the mask may pose a problem for welding damping means on its surface: any final alteration to the surface of the mask may cause rejection of the complete masking device. Moreover, when welding damping elements to the edges of the mask, welding sputter may occur and close off the holes on the central surface of the mask, which would also cause the whole masking device to be rejected.
It is an object of the said invention to provide a cathode-ray tube comprising a masking device for a colour cathode-ray tube comprising simple, inexpensive damping means which are easy to fit without leading to deterioration of the mask surface and equally suitable for the tensioned mask structure as for a mask structure formed by pressing.
To do this, the cathode-ray tube according to the invention comprises:                a colour selection mask in the form of a metal sheet, adapted in order to be fastened to a substantially rectangular support frame and comprising one pair of short parallel sides and one pair of long parallel sides, the frame/mask assembly being placed inside the glass front face of the tube,        means of damping vibrations of the frame/mask assembly, placed on each side of at least one pair of parallel sides of the frame,the said damping means being characterized in that they comprise at least one mass connected to the side of the frame by a connection having a mechanical clearance between the facing surfaces of the mass and of the frame.        